This invention relates to stable aqueous latexes of interpolymers of a monovinylidene aromatic monomer and an open chain aliphatic conjugated diene and to the use of such latexes in paper coating compositions.
Stable aqueous dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful, both alone and in various formulations, as coatings and impregnants for various types of substrates. A wide variety of latexes of differing homopolymeric and copolymeric composition (such as styrene-butadiene copolymers, acrylic homopolymers and copolymers, vinylidene chloride homopolymers and copolymers, etc.) have been developed having specific chemical and/or mechanical properties for particular end use applications. For example, aqueous interpolymer latexes resulting from the emulsion polymerization of monovinylidene aromatic monomers, such as styrene; diolefins, such as butadiene; and monoethylenically unsaturated carboxylic acids, such as acrylic acid; are known to be particularly useful as film-forming binders for pigments in paper coating applications. See, for example, U.S. Pat. Nos. 3,399,080 and 3,404,116. Such emulsion polymerizations optionally employ conventional seeding procedures for optimum control of the polymerization and to obtain maximum product uniformity (e.g., narrow particle size distribution). The resulting interpolymers are generally film forming at room temperatures (i.e., from about 20.degree. to about 25.degree. C.) and are sometimes referred to as "soft" paper coating binders. Pigmented paper coatings employing such prior art "soft" interpolymer latexes are generally characterized by excellent pigment binding, often expressed in terms of wet pick and dry pick test results. However, such coatings are also characterized by somewhat limited gloss development, ink receptivity and stiffness. Naturally, improvement of the stiffness, gloss, and ink receptivity in such coatings is an ever-present goal of the industry.
Attempts have been made in the prior art to provide improved gloss and ink receptivity as well as improved stiffness while retaining adequate binding and smoothness. Such attempts have included the use as binders of blends of soft (or elastomeric) latexes with hard (or plastomeric) latexes. See, for example, U.S. Pat. No. 3,281,267. In addition, the use of plastic pigments in conjunction with, or as a replacement for, conventionally employed mineral pigments has been suggested. While such approaches have provided a measure of improvement in gloss, attendant sacrifices in other desirable coating properties (e.g., binding strength, wet strength, etc.) have been encountered.
More recently, the use of "hard" or "stiff" latex binders, which themselves impart a measure of stiffness and gloss to the coated paper articles, has been suggested for attainment of high gloss, stiff paper coatings. Such coatings generally employ latex binders of what are known as hard polymers. Such hard polymers generally have minimum film-forming temperatures in excess of normal room temperatures (e.g., 100.degree. F. or higher). Accordingly, film formation to achieve binding of the pigment particles to each other and to the paper substrate is accomplished for such coatings at elevated temperatures. Such temperatures are controlled to insure only limited (i.e., incomplete) deformation of the hard polymer particles in order to achieve both binding of the pigment particles and the desired coating gloss. See, for example, U.S. Pat. Nos. 3,583,881; 3,634,298 and 3,873,345. Unfortunately, however, such prior art processes for high gloss paper coating are deficient in that (a) the coatings are characterized by relatively low binding strength and (b) the properties of the resulting coated paper product (such as gloss, ink receptivity, stiffness, smoothness and pigment binding strength) are generally highly sensitive to even relatively small variations (e.g., 5.degree. to 10.degree. C.) in the temperature of the film-forming step (e.g., drying or hot calendering). While sophisticated processing equipment and control systems can be employed to minimize processing temperature variations, such systems are expensive and are generally incapable of completely eliminating such temperature variations and the corresponding variations in product quality.
In view of the foregoing deficiencies in the binders of the prior art paper coatings, it would be highly desirable to provide an improved stiff binder which essentially eliminates these deficiencies, particularly those related to gloss, ink receptivity, binding strength and desirable combinations thereof and especially which reduces the sensitivity of such properties to processing temperature variations.